The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Vehicles include an internal combustion engine that generates drive torque. More specifically, an intake valve is selectively opened to draw air into the cylinders of the engine. The air is mixed with fuel to form a combustion mixture. The combustion mixture is compressed within the cylinders and is combusted to drive pistons within the cylinders. An exhaust valve selectively opens to allow the exhaust gas to exit from the cylinders after combustion.
A rotating camshaft regulates the opening and closing of the intake and exhaust valves. The camshaft includes a plurality of cam lobes that rotate with the camshaft. The profile of the cam lobe determines the valve lift schedule. More specifically, the valve lift schedule includes the amount of time the valve is open (duration) and the magnitude or degree to which the valve opens (lift).
Variable valve actuation (VVA) technology improves fuel economy, engine efficiency, and/or performance by modifying a valve lift event, timing, and duration as a function of engine operating conditions. Two-step VVA systems include variable valve assemblies such as hydraulically-controlled switchable roller finger followers (SRFFs). SRFFs enable two discrete valve states (e.g., a low-lift state or a high-lift state) on the intake and/or exhaust valves.
Referring to FIG. 1, a hydraulic lift mechanism (i.e., an SRFF mechanism) 10 is shown in more detail. Those skilled in the art can appreciate that the SRFF mechanism 10 is merely exemplary in nature. The SRFF mechanism 10 is pivotally mounted on a hydraulic lash adjuster 12 and contacts the valve stem 14 of an inlet valve 16 that selectively opens and closes an inlet passage 18 to a cylinder 20. The engine inlet valve 16 is selectively lifted and lowered in response to rotation of an inlet camshaft 22 on which multiple cam lobes (e.g., low-lift cam lobe 24 and high-lift cam lobe 26) are mounted. The inlet camshaft 22 rotates about an inlet camshaft axis 28. Although the exemplary embodiment describes the SRFF mechanism 10 operating on the engine inlet valve 16, those skilled in the art can appreciate that an SRFF mechanism may operate similarly on an exhaust valve 30. Such as the configuration required to enable an HCCI operating regime.
A control module transitions an SRFF mechanism from a low-lift state to a high-lift state, and vice versa, based on demanded engine speed and load. For example, an internal combustion engine operating at an elevated engine speed, such as 4,000 revolutions per minute (RPMs), typically requires the SRFF mechanism to operate in a high-lift state to maintain valvetrain stability.
Hydraulic cam phaser movement and positioning is achieved by controlling the flow of oil to the cam actuator, such as a phaser. The flow control is done with a valve capable of supplying oil to a volume on one side of a vane in a phaser while simultaneously providing a path for the volume on the other side of the vane to vent or return to a tank. The rate of oil flow is a function of the area of the flow port that is exposed. The control of the flow is achieved by varying the amount of force applied to the valve spool, which may be obtained from a solenoid.
As mentioned above, a two-step SRFF may have a maximum speed of operation in the low-lift state. The inertia of the mechanism operating the low-lift state above the maximum engine speed will exceed the spring force, which maintains the contact between the SRFF and the cam lobe. The resulting separation between the SRFF and the cam may eventually fatigue the parts and cause damage. Preventing damage increases the durability of the engine.